Heat reflector

ABSTRACT

Method and apparatus for producing columnar castings by precision investment casting techniques, wherein a refractory reflector is positioned in the furnace behind the molds contained therein to make it possible to control the unidirectional temperature gradient more closely and provide more pieces per mold.

United States Patent Hein et al.

[ 1 Aug. 1, 1972 [54] HEAT REFLECTOR [72] lnventors: Frank J. Hein,Minerva; Donald G.

Fleck, Alliance, both of Ohio [73] Assignee: TRW Inc., Cleveland, Ohio[22] Filed: Nov. 12, 1970 [21] Appl. No.: 88,608

52 US. Cl. ..164/127, 164/60, 164/129, 164/353, 164/361, 164/338,249/111 51 Int. Cl ..B22d 25/06, 822d 27/04 [58] Field of Search..l64/60, 122, 125, 127, 129, 164/338, 353, 361; 249/111 [56] ReferencesCited UNITED STATES PATENTS 3,515,205 6/1970 Wickstrand ..164/3533,248,764 5/1966 Chandley 164/ l 27 3,417,809 12/1968 Sink ..164/1273,627,015 12/1971 Giameietal ..l64/60 Primary Examiner-J. SpencerOverholser Assistant Examiner-John E. Roethel Attorney-Hill, Sherman,Meroni, Gross & Simpson 5 7] ABSTRACT Method and apparatus for producingcolumnar castings by precision investment casting techniques, wherein arefractory reflector is positioned in the furnace behind the moldscontained therein to make it possible to control the unidirectionaltemperature gradient more closely and provide more pieces per mold.

5 Clains, 3 Drawing Figures PATENTEDAU I I972 3.680.625

HEAT REFLECTOR BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention is in the field of precision investment casting toproduce columnar structures, and is particularly involved with reflectorelements for mold clusters which make it possible to employ more moldson a given cluster than has heretofore been the practice.

2. Description of the Prior Art In recent years, there has been asubstantial amount of development work done on the directionalsolidification of castings to produce columnar grain structures. It hasbeen found that such castings have superior elevated temperatureperformance in gas turbine engines than castings produced with equiaxedgrain structures.

Columnar structures are generally produced by positioning a doublyopen-ended ceramic mold on a chill block composed of copper or otherhighly heat conductive material. The mold structure is positioned withina furnace, usually heated by selectively energizable induction heatingcoils and provided with a susceptor which radiates the heat at thecluster of molds within 2 the furnace. The molds are preheated to atemperature at least as high as the solidus temperature of the metal tobe cast, and the molten metal is then cast into the molds.solidification proceeds upwardly from the copper chill block and iscontrolled by a variety of means, including selective deenergization ofthe induction heating coils to produce a unidirectional temperaturegradient throughout the mold during solidification.

Heretofore, the number of molds which could be utilized in a givencluster has been limited. This, of course, is undesirable since thegreater the number of pieces which can be obtained per castingoperation, the more economical is the process. Basically, the number ofpieces which can be produced from a mold cluster is limited by thegeometry of the part and the size of the induction heating coil. Inpractice, however, it has been found that the overall geometry of themold and the proximity of one mold structure to another have an effecton the ability of the system to produce acceptable grain structures. Ithas been found necessary on some configurations, for example, to thickenthe mold at the top as much as one inch to assure proper solidificationcharacteristics. These drawbacks limit the number of molding cavitieswhich can be fed from a common source and solidified under conditions ofunidirectional cooling to produce columnar structures.

SUMMARY OF THE INVENTION This invention provides improvements in thefield of precision investment casting and, more particularly, in thespecific field of producing columnar grain structures in castings.Specifically, we have now found that by positioning a ceramic reflectorelement within the furnace assembly, the interaction between theindividual molds in the mold cluster is significantly reduced and moremolds can be put on a cluster thereby increasing the yield of theprocess, and a higher percentage of the castings result in the desiredcolumnar grain structure.

The reflector is positioned between the central portion of the mold andthe molding cavity so that the molding cavities are disposed between thereflector and the radiating inner wall of the furnace, which is usuallya graphite susceptor. The ceramic reflector may be separately introducedinto the molding assembly or, more preferably, it may be an integralpart of the mold produced at the same time as the remainder of thecluster by the usual precision investment mold making processes.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention will be readily apparent from the following descriptionof certain preferred embodiments thereof, taken in conjunction with theaccompanying drawings, although variations and modifications may beeffected without departing from the spirit and scope of the novelconcepts of the disclosure, and in which:

FIG. 1 is a plan view of a pattern cluster which can be used for makingthe mold assemblies of the present in- VCIIIIIOII;

FIG. 2 is a cross-sectional view taken substantially along the lineII-II of FIG. 1; and

FIG. 3 is a view partially in cross-section and partially in elevationof a mold assembly and furnace as- 5 sembly of the type with which thepresent invention is involved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A pattern assembly forproducing shell type investment molds of the present invention isillustrated at reference numeral 10 in FIG. 1. While the particularpattern as shown in the drawings is designed for the production of fourmolds, it should be understood that any number of mold assemblies can beemployed and one of the advantages of the present invention is that themold assemblies can be placed closer together in the cluster than hasheretofore been commonplace. Individual patterns 11 through 14 composedof wax or other pattern material are spaced about a central pouringbasin-forming portion 15 is connected to the upper ends of the patterns11 through 14 by means of upper runners 17 through 20, respectively.Similarly, the bottom ends of the patterns 11 through 14 are connectedto the base of the sprue-forming portion 16 by means of radiallyextending runners, two of which identified at reference numerals"-2l and22 are visible in FIG. 2. It will be understood that in keeping withordinary investment casting procedures, the patterns 11 through 14 maybe made in individual pattern molds and thereupon connected to therunners and the pouring basin forming portion 15 and the sprue-formingportion 16 by means of heat welding or solvent welding.

The pattern assembly shown in FIGS. 1 and 2 also includes areflector-forming portion 23 which may consist of a thin sheet of waxwhich is suitably secured to the bottom runners 21 and 22 as well as thebottom runners feeding the patterns 12 and 14. Typically, the sheet 23may be about 0.1 inch in thickness.

The pattern cluster shown in FIGS. 1 and 2 is then used to form a moldcluster through conventional precision investment mold-makingtechniques. One such method involves coating the wax pattern assembly bydipping it in an aqueous ceramic slurry having atemperature about thesame asthat of the pattern material to fonn a refractory layer of a fewmils in thickness. A typical slurry may contain ceramic material such aszirconium oxide, a binder such as colloidal silica and a thickener andlow temperature binder such as methyl cellulose. The methyl layer whilestill wet is then dusted with small particles (-40 to 200 mesh) of arefractory glass composition such as that known as Vycor which is afinely divided, high silicon oxide glass containing about 98 percentsilica and a small amount of boric acid, together with traces ofaluminum, sodium, iron and arsenic. The pattern with the dusted wetrefractory layer on it is then suspended on a conveyor and moved to adrying oven having a controlled humidity and temperature, thereby dryingthe coated pattern assembly adiabatically.

The steps of dipping, dusting and adiabatic drying are then repeatedusing air at progressively lower humidities for succeeding coats. Forexample, the first two' coats canbe dried with air having a relativehumidity of 45 to 55 percent. The third and fourth coats can be driedwith a relative humidity of 35 to 45 percent, the fifth and sixth coatswith a relative humidity of 45 to 55 percent. The third and fourth coatscan be dried with a relative humidity of 35 to 45 percent, the fifth andsixth coats with a relative humidity of to 'percent, and the final coatwith a relative humidity of l5 to 25 percent.

The first layer is preferably applied to a thickness of 0.005 to 0.020inch, and the fine refractory particles are dusted onto the wet layerwith sufficient force to embed the particles therein. It is preferredthat the dusting procedure used provide a dense uniform cloud of fineparticles that strike the wet coating with substantial impact force. Theforce should not be so great, however, as to break or knock off the wetprime layer from the pattern. This process is repeated until a pluralityof integrated layers is obtained, the thickness of the layers each beingabout 0.005 to 0.020 inch.

. After the mold has been built up around the pattern assembly, thepattern material can be removed by heat and then the green mold is readyfor firing. Generally, firing temperatures on the order of l,500 to1,900? P.

. are used. The resulting shell molds are hard, smooth and relativelypermeable, and have a thickness on the order of one-eighth to one-fourthinch.

The resulting mold cluster produced from the pattern assembly 10 isshown at reference numeral 30 in FIG. 3. The mold assembly 30 isdisposed within a furnace having a refractory outer wall 31 about whichone or more induction heating coils 32 are disposed. Located within thewall 31 is a susceptor 33 composed of graphite or the like which servesto deliver radiant energy to the molds. The top of the furnace is closedby means of a top plate 34 composed of refractory material, and a funnel35 is provided to deliver molten metal to the casting cavities.

The mold assembly 30 itself contains a pouring basin 36 and acylindrical sprue portion 37 extending downwardly therefrom. The pouringbasin 36 communicates with the interior of the casting molds, two ofwhich have been identified at. reference numerals 38 and 39 in FIG. 3.Runners 40 and 41 are used to deliver molten metal from the commonsource into the casting cavities of the respective molds.

The mold assemblies are open ended and their bottom ends are positionedon a chill block 42 composed of copper or other highly heat conductivematerial. If desired, a circulating fluid may be passed through thechill block 42 to increase the rate of heat transfer.

The mold assembly also includes a refractory reflector 43 which in theform of the invention illustrated in FIG. 3 consists of acontinuousannulus of ceramic mold fonning material with a hollow center.Alternatively, the reflector 43 can consists of a preformed ceramicmaterial which is placed in the mold prior to pouring and it need not becontinuous. In other words, a plurality of ceramic bafi'les can bepositioned closely adjacent the individual molds, the width of thebaffles being at least as large as the projected widths'of the moldassemblies with which they are associated.

in operation, the molding assembly is operated under vacuum conditions,and the molds are heated to a temperature above the solidus temperatureof the metal to be poured in the mold. The metal is melted and cast intothe mold and the temperature of the mold is gradually reduced in orderto obtain unidirectional solidification from the chill block 42 upwardlyto the top of the casting. One convenient means of doing this is toprogressively deenergize individual coils making up the inductionheating coil 32 .so that as solidification I proceeds upwardly, aunidirectional temperature gradient exists longitudinally of the mold,and a columha grain structure having a longitudinal orientation isproduced.

It has been found that through the use of the reflector elements of thepresent invention, it is possible to employ more molds in the cluster.than heretofore used without adverse interaction occurring between themolds. It has also been found that the grain structure of the castingsis actually improved primarily we believe, because the reflectingelements help to produce a more carefully controllable temperaturegradient within the mold assembly.

We claim as our invention:

said wall, a susceptor disposed inside said wall-and adjacent thereto, acasting mold assembly comprising a plurality of circumferentially spacedceramic molds enclosed by said wall, a common feed means disposedcentrally of said molds for introducing molten metal radially into eachof said molds, and refractory reflector means separate from said ceramicmolds disposed in closely spaced relation to said molds on the sidesthereof opposite to said susceptor.

2. The apparatus of claim 1 in which said reflector is integral withsaid mold.

3. The apparatus of claim 1 which includes a surface of high heatconductivity upon which said molds rest.

4. The apparatus of claim 1 in which said reflector means is in the formof an annulus surrounding said common feed means.

5. The method of producing columnar castings which comprises positioninga plurality of spaced, openended ceramic molds on a highly heatconductive surface within a furnace including a susceptor which radiatesheat at said molds, preheating said molds to a temperature above thesolidus temperature of the metal to be cast, positioning a ceramicreflector inwardly of said molds so that such molds are disposed betweensaid reflector and said susceptor, said reflector being separate fromsaid ceramic molds and being positioned to reflect heat radiated fromsaid susceptor to portions of said molds which would otherwise bescreened from such radiated heat, pouring molten metal into said molds,and providing a unidirectional 5 temperature gradient within said moldsduring the solidification of the metal therein.

mun-STAT S S'ATENT F CERTIFICATE-OF;C-QRRECTION Peerin 80,6 5 na August1,1972

- m mas Frank Hein z Donald G. Fleck It is vcertified that error appearsin the above-identified patent and that said Letters Patent are hereby.corrected as shown 'below:

Column 2, line 41, after "151': insert -.-and-a sprue-f orming portion16. The pouring basin-forming portion 'l5-- igned and sealed this 22ndday of May, 1973.

[SEALl EDWARD-MILETCHERQJR.7- ROBERT GOTTSCHA'LK Attesting Officer 1Commissioner of Patents FORM PC4050 (19-69)

1. An apparatus for precision casting comprising a furnace wall, aninduction heating coil disposed around said wall, a susceptor disposedinside said wall and adjacent thereto, a casting mold assemblycomprising a plurality of circumferentially spaced ceramic moldsenclosed by said wall, a common feed means disposed centrally of saidmolds for introducing molten metal radially into each of said molds, andrefractory reflector means separate from said ceramic molds disposed inclosely spaced relation to said molds on the sides thereof opposite tosaid susceptor.
 2. The apparatus of claim 1 in which said reflector isintegral with said mold.
 3. The apparatus of claim 1 which includes asurface of high heat conductivity upon which said molds rest.
 4. Theapparatus of claim 1 in which said reflector means is in the form of anannulus surrounding said common feed means.
 5. The method of producingcolumnar castings which comprises positioning a plurality of spaced,open-ended ceramic molds on a highly heat conductive surface within afurnace including a susceptor which radiates heat at said molds,preheating said molds to a temperature above the solidus temperature ofthe metal to be cast, positioning a ceramic reflector inwardly of saidmolds so that such molds are disposed between said reflector and saidsusceptor, said reflector being separate from said ceramic molds andbeing positioned to reflect heat radiated from said susceptor toportions of said molds which would otherwise be screened from suchradiated heat, pouring molten metal into said molds, and providing aunidirectional temperature gradient within said molds during thesolidification of the metal therein.